Resistance-capacitance oscillator



June 29, 1948. M. ARTZT RESISTANCE-CAPACITANCE OSCILLATORS 2 Sheets-Sheet l Filed June 21, 1943 VOL THG'E 600v f flMPL Zhweutor H 7' TE/V- OUTPUT U070 13 HMPL IF IE/B Gttomeg Patented June 29, 1948 RESISTANCE-CAPACITANCE OSCILLATOR I Maurice Artzt, Princeton, N. 5., assignor to Radio Corporation of America, a corporation of Delaware Application June 21, 1943, Serial No. 491,613

14 Claims.

The invention relates to resistance-capacitance oscillators and has particularly to do with improvements in oscillators of that type which aim at frequency stabilization as well as amplitude stabilization.

As is well known, resistance-capacitance oscillators have a wide field of utility. For example, they are preferable to oscillators which require tuned inductance-capacitance circuits when a Wide range of frequencies is to be obtained. Resistance-capacitance oscillators are also preferred for generating frequencies in the sub-audio range and all higher frequencies up to, say, 100 kilocycles. Oscillators of the resistance-capacitance type are also preferred in certain applications because of the purity of the sine waves which they generate. In actual tests of certain apparatus which has been built in accordance with my invention for working in the range of 60 to 300 cycles, a fixed frequency stability of one part in 10,000 parts has been obtained.

Another very useful application of my invention is in the generation of tone frequencies for facsimile transmission and in other communications systems requiring sub-carrier frequencies.

My invention will now be described in more detail, reference being made to the accompanying drawings, in which:

Fig. 1 shows a capacitance-resistance oscillator, hereinafter referred to as a C. R. oscillator, of a well-known type using a so-called Wien bridge circuit;

Fig, 2 shows a circuit diagram similar to that of Fig. 1 but incorporating therein certain im-- provements to which my invention relates;

Fig. 3 shows a graph of frequency vs. voltage gain in an oscillator amplifier system such as that of Fig. 2;

Fig. 4 shows a modification of my invention in which automatic amplitude control is provided. The circuit of Fig. 4 also embodies a switch for selecting any one of three ranges of frequencies to be generated. v

Fig. 5 shows still another modification of my invention similar in some respects to Fig. 4, but suitable for operation in a higher frequency range; and

Fig. 6 shows a preferred circuit arrangement that is, they include a cathode 3, an

anode 4 and a control grid 5. The'anodes 4 are supplied with suitable positive potentialfrom a D. C. source, indicated conventionally as +B, this source being fed through individual anode resistors 6. l .Individual cathode resistors I are shown connected between each cathode and ground. These resistors l are shunted by capacitors 8. The input circuits in the two tubes include grid leak resistors 9a and'sb respectively which are connected between the individual grids '5 andground. The grid 5 in tube 1, however, is controlled by feedback energy through a circuit from the anode 4 in tube 2 and thence through a resistor It and variable capacitor 1 l. Another variable capacitor I2 is in shunt with resistor 9a. The outputcircuit for tube 2 includes a load resistor l3 which is coupled to the anode 4 in tube 2 by means of a capacitor It. The grid 5 in tube 2 is controlled by voltage variations across resistor 91) due to the potential on the anode 4 in tube 1, the control energy being passed through a coupling capacitor [5. 1 l

The operationof the circuit arrangement of Fig. 1 will be explained even though this circuit represents prior art, but in order to provide a background for the improvements which are to be he'reinafter set forth and claimed.

The Wien bridge oscillator functions with a zero shift between the inputside of tube 1 and the output side of tube 2. In order to eliminate according to my invention in which an R. C.

oscillator is provided for phase correction with constant frequency of output.

Referring first to Fig, 1, I show therein asocalled Wien bridge oscillator of the prior art in which two discharge tubes I and 2 are provided. Each of these tubes may be of the triode type,

phase shift, the capacitors H and I2 are usually adjusted to equal values and the two resistors 9a and I0 are also made equal. The tubes tend to oscillate at the periodic frequency of the network which includes elements l0, ll, l2, and 9a. The. output frequency also depends in lesser degree upon the anode impedance of tube 2 together with anode resistor 6. For frequencies below theperiodic frequency, the output voltage of the network is advanced in phase. For frequencies above the periodic frequency, the output in retarded in phase. The circuit will tend t oscillate at the frequency requiring the lowest gain where the phase displacement throughout the entire circuit is 0, 360 or some multiple thereof. The amplification in the tube circuit is preferably limited tovery little more than the loss in the network. The normal loss in a Wien bridge circuit is 3 to 1. For self-oscillation, therefore, an amplification factor of slightly more than 3 is required. Under these conditions, har monies are suppressed and a substantially pure sine waveis obtained.

Referring now to Fig. 2, I show therein an R. C. oscillator in which parts similar to those of Fig. 1 are designated with like reference numerals. It will be noted that the grid 5 in tube 2 is grounded. Also there is a common cathode resistor IS in place of the independent cathode resistors l of Fig. 1. The feedback circuit between the anode of tube 2 and the grid of tube l includes an adjustable potentiometer ll, the tap N3 of which may be set thereon in a position to deliver an optimum feedback voltage through the network on the input side of tube I, this network including resistors I and 9a capacitors II and I 2.

In the operation of the circuit arrangement of Fig. 2 it will be observed that each tube is biased toward cut-01f by a conductive state in the other tube. This is true because thecommon cathode It operates to maintain the same instantaneous cathode voltage in the two tubes.

The grid in tube 2 is,- however, maintained at ground potential, while grid'5 in'tube l' responds to the feedback energy derived from the voltage drop in potentiometer ll'with variations of anode potentialin tube 2. The time constant network connected b etween th e gridof tube l and ground determinesthe frequency at which tube 1 shall oscillate. These oscillations in turn control the oscillations in tube 2. The output circuit from this tube'is the same as that shown in Fig. l.

4 The relation between frequency and gain for the oscillator circuit arrangement shown in Fig. 2 is shown in Fig. 3; I A 1 There are two definite reasons why this low gain amplifier will have a more stable frequency than a high gain circuit with A. V. C. and feedback. First, the network is operated at very low amplitudes, and the heat generated in it is small. The temperature effect on resistors and capacitors is greatly reduced and the resulting improvement of frequency drift with warm-up is very noticeable.- Second, the output resistor of the second tube is very-small compared to the impedance of thenetwork. Therefore, changes inthe characteristics of the drive tube will cause lesschange in the phase angle between current and voltage at the network input than when this plate resistor is high. The effect on frequency of changes in tube constants. with age or line voltage-is, therefore, reduced.

Fig. 1 shows a development of the basic circuit arrangement of Fig. 2 which is suitable for Ohtaining a wide range of "frequencies. One of the most fruitful applications for R. C. oscillators is that of usingthe same to replace inductance-capacitance oscillators. quencies to be obtained may readily be extended all the way between cycles and 20,000 cycles. It is preferable, however, to arrange for switching different resistive impedances into the time constant network for the tube I so as to provide say for three decimal steps covering the three following ranges:

20" to 200 cycles 200 to'2,000 cycles 2,000 to- 20,000 cycles.

Accordingly, the'switches Hand 20 as shown in Fig. 4 may be ganged together. Also it ispref The range of freand variable ganged I era'ble to gang together the two variable con- 3 which to ground the lower electrodes of condensers H and Ma. The frequency is inversely proportional to the capacitance and not to its square root as in an inductance-capacitance oscillator. A 10 to 1 frequency spread in an R. C. oscillator is just as easy to obtain as a 3 t0 1 spread in an L. C. oscillaton- The following parameters may be taken as an example of what may be used in the circuit arrangement of Fig. 4 to produce a given frequency: Let the tuning condensers l I and [2 have avalue of 500 mmrd; The tuning range of 20 to 200 cycles is available by setting switches l9 and 2 0 earnestness a, whereat resistors 2i and 22 are included in the network. These resistors should have a value of the order of 16 megohms in order-'topfod'uce this range of frequencies.

By settin switches is and 20 to switch points b thereby to include resistors 23 and 20 in the network, these resistors having avalue of the order of f1 megohms, the" range offrequencies to be ob'ta will be from 200'to2L000 cycles.

'In the third position of switches 19 and '20, that'is'to switch points 0, where'resistors 25 and 26 are included, and where these resistors have a value of the order of 160,000 ohms, a range of frequencies of 2,000 to 20,000 cycles is obtained. The capacitors ,l'l and'ii are, of course, adjusted within their variable range of 50th 500 mmfd. in order to set the frequency adjustmentto any desired value. Thereis no need, however, to set these tuning condensersto the extreme low 'end of their adjustment. The'1 6 megohm value used for resistors 2i and 2?. is high enough so that great-care inshielding is requiredto reduce power frequency hum to a satisfactory level.. If the variable condensers l and H were to have a range of 1,000-to 1*00 mmfd then the values of resistors 26, 23, and 25' would preferably be of the order of 8 megohms, 0.8 megohm, and 80,000 ohms respectively, corresponding resistors 22, 24, anl'd'Zli having like values. The selection of resistors ofthese values would have an advantage in that the possibility of hum pick-up would be reduced. Inthe high frequency range, the circult wouldpresent' a greater load to the driver, but without any disadvantage so far as frequency st'abilit'yis concerned.

The-utilization circuit illustratively shown in Fig." 4 includes an amplifier 00 and anattenuator t l. The input circuit for the amplifier 60 includes a grounded resistor 02, the floating end of which is coupled across capacitor 63 to the anode l in tube 2. The amplifier 50 is, therefore, controlled by variations in the anode potential in tubes.

An A. V. C. circuit is shown to derive its control from the output side of amplifier 60 which is coupled across capacitor M to the cathode'of a diode rectifiertube 27. The energy component which is rectified by tube 2'! is supplied as a negative bias to the grid. 5 of tube 2. Tube 2 has an inputcircuit which includes a grid resistor 28' connected'between the grid 5 and ground. This resistor is shunted by a capacitor 29. Across the power supply terminals is a series resistance comprising resistor 30a and potentiometer 3%.

through resistor 30 to the cathode of tube 2?.

This threshold bias holds the amplitude of oscillation to'a low value so as to minimizeharmonic distortion. Preferably, the overall gain should be so set that without the functioning of the A. V. C. circuit, this gain will be somewhat higher than needed to bring about Oscillation at the frequency of greatest network loss.

With reference to the Operation of either of the circuits shown in Fig.2 and Fig. 4, it will be seen that either of these circuits represents a novel drive amplifier for use with zero phase shift networks. Although two tube elements are used, a circuit such as either of these is in reality a single stage amplifier with low gain and with its output in phase with the input. Being direct connected, the frequency response extends to zero frequency or D. C., and, therefore, there is no possibility of phase shifts at low frequencies due to coupling circuits or bypass capacitors.

Thefirst tube acts as a cathode follower on the input from the network, and in turn changes the cathode to ground potential of the second tube, instead of feeding into the second grid. The phase shift from input to output of the two tubes is zero for all frequencies up to that point where tube capacities begin to take effect. As this occurs somewhat above 20,000 cycles for the circuit as normally used, the range of 20 to 20,000 cycles is easily covered by-this amplifier.

Proper selection of cathode and plate resistors allows the gain to be made just sufficient to drive the bridge, but with no excess to overdrive and causesquare Waves. Under this condition the harmonic content of the wave is very low.

With such a small margin of gain over that required, any large deviation from a true balance in the network will seriously alter the amplitude of oscillation. The gain required for the network is 3 if both resistors are exactly equal and both capacitors are likewise equal. Butif the variable capacitors are unbalanced, or do not track over the useful range, it will vary considerably from 3. A serious unbalanced can cause enough network loss to stop oscillation, or overdrive and give distortion. For this reason the resistors should be well matched and the two variable capacitors should track very closely. When this is done it will generally be found that the stray grid to ground capacity can be offset by putting a small trimmer condenser Na in shunt with the plate capacitor; setting this to a value where the amplitude of oscillation does not change in sweeping the capacitors over their entire range. Once set for any one range, changingto the other ranges should not affect this balance.

There is one precaution that should be especially noted for this type'of oscillator in the low range of 20 to 200 cycles. The high v'alueof grid leak used, 8 megohms or more, makes the shielding requirements for the network and first tube severe. Any hum pick-up will be readily apparent, and if the oscillator is set near 30, 60, 90, 120, or 180 cycles, it will tend to lockup or synchronize itself to the hum, Accurate test work in this frequency range is impossible without good shielding around all vulnerable parts. This is not necessarily true of fixed frequency oscillators or where the network resistors are under one megohm.

Referring now to Fig. 5, I show still another embodiment of my invention, a characteristic of which is that the circuit arrangement is particularly well adapted for frequencies up to 200 kc. Like the circuits of Figs. 2 and 4, Fig. 5 is also direct coupled for response at zero frequency, but at the same time, the range is very much extended to higher frequencies. A balanced D. C. amplifier is used asthe input stage consisting of tubes 3| and 32. The circuit arrangement ofthis amplifier is fundamentally the sameas that shown and described in my'Patent No. 2,310,342, dated February 9, .1943.

A pair of serially connected equivalent resistors 45, 46 is disposed across the terminals, v., -l50 v. of a, power. source. The junction between resistors 45 and 48 is connected through a load resistor 4! to the cathode 40 in tube 32. A peak voltage regulator device 41 of any well-known type is preferably connected across the power supply terminals.

The second stage of this oscillator circuit comprises the triode tube 42, from the anode of which feedback potential is derived for application to the input circuit of tube 3|. The same selective switching arrangement is used in Fig. 5 as in Fig. 4, although preferably a greater choice of network resistors is provided by adding a fourth position to the switches |9and 20 and by connecting the switch points d individually to resistors 25a andZfia.

The input circuit for tube 42 includes the adjustable resistor, 4|. The output circuit includes the anode resistor 43 and resistor 46 which is connected between the cathode of tube 42 and ground, Bypass condenser 48 is in shunt with resistor 45.

As explained in-my Patent No. 2,310,342, aforementioned, thevoltage of the power source, whatever it mayhe, would be equally divided across resistors 45 and 45, except for the shunting action of the tubes 3!, 32 and 42. But when tube 3| draws current, the potential drop in resistor 38 connected to the anode 35 biases the grid 34 more negatively and increases the impedance of tube 32. At this moment, therefore, the anode current for tube 3| i for the most part fed through resistors 45, 4|, and 38 in series. The potential drop in resistor 4| biasesthe grid in tube 42 negatively and reduces the potential drop in resister 43. A more positive feedback potential is applied to switch-blade Hi and through the connected resistor to the upper electrode of condenser i In operationthe tubes 3| and 32 behave as class A amplifiers. Consider first the condition where the feedback potentials applied to the grids 33 and 34 are such as to cause'the two tubes to draw current of like amplitude. Then the voltage across resistor 4| is substantialy zero because the value of resistor 45 is substantially equal tothe sum of theimpedances through resistors 31 and 33 plus the space path of tube 3|. Now by making tube 3| draw more current than tube 32 the voltage drop through resistor 4| causes the grid of tube 42 to be negatively biased. The overall amplification factor of the, circuit is represented by the value of the maximum potential drop across resistor 43 in relation to the voltage swing applied to the grid 33; The values of resistors 4| and 43 are, therefore, suitably adjusted so that the amplification factor is just slightly larger than the network loss from resistor 43 back to the grid 33. At the frequency where the network phase shift is 0 the circuit will then oscillate and deliver a sine wave output.

At one part of the cycle, the grid 34 in tube 32 is driven more positive and the current flow to the anode 36 in tube 32 traversesresistors 4| and 46. thereby imposing a positive bias on the grid of tube 42 and causing this tube to be more conductive' The potential drop in resistor 43 is increased. All of the conditions are fulfilled,

therefore, for providing an oscillatory output energy from the anode of tube 42 both for feedback purposes and for any desired utilization.

The two resistors 3'! and 33 are preferably made equal. Resistor 31 is connected between ground andthe cathode 39in tube3'l Resistor 38 is connected between the anode 35 in tube 3! and the cathode tll in-tube 32. The adjustable load resistor li preferably has a relatively low value, say'o'f the order of 600 ohms, so that the overall gain of the first stage is equal to slightly more than 3. The output from tube 42 is obtained by operation at a low grid swing and with very low bias voltage, the bias being the potential drop in resistor M. This output-resistor is made low in comparison with the lowest of the network resistors connected to switches l9 and 2B. The output through capacitor 50 is intended to be coupled to the grid of a subsequent tube stage and as such will present no appreciable load to tube 42, This choice of a low ohmic value for resistor is is intended to prevent phase shift. For example, it may be of the order of 1,000 ohms or less. Under these conditions, the phase shift from the grid 33 in tube 3i to'the anode in tube #32 is very small for all frequencies below 200 kc. and the gain is adequate for oscillation.

If the gain of the first stage is made greater by increasing the ohmic value of resistor l i, then the output resistor d3 can be lowered to drive a stiffer network. Under this condition the first stage may tend to fall'in gain at the higher frequencies due to the overall capacitance-of the system between resistor 45 and ground. This is recovered, without appreciable phase shift, by the addition of a small capacitor 44 in shunt with cathode resistor 31. Not over 30% of the gain reduction can be made up in this manner without introducing considerable phase shift, resulting in poorer accuracy of calibration.

Fig. 6 shows a circuit diagram of another modification of my invention which provides a solution to certain problems of phase correction. It is apparent to those skilled in the art that any deviation from linearity, or any phase shift with in the tube circuit will change the frequency of oscillation. The phase change is usually the more difficult to correct, especially in the high frequency spectrum, and it is for this reason that great stability in a low frequency oscillator is more easily attained than in a high frequency oscillator.

The circuit arrangement of Fig. 6 comprises discharge tubes land 2 which are arranged with triode electrodes the same as the equivalent tubes in Fig. 2. An auxiliary tube is provided, the circuit of which functions as a compensating inductance. This will be hereinafter explained in more detail. Tube 5! is preferably of the triode type. Its cathode circuit impedance includes a resistor R5 and an inductance L leading to ground. The anode circuit is common to that of tube 2, that is, it contains a common anode resistor R4.

On the input side of tube lis a non-inductive time constant network comprising grid resistor Rd incircuit between the grid 52 and ground, capacitor CE and resistor R2 in circuit between the grid 52 in tube I and the anode 55 in tube 2, and capacitor 02, the latter being connected from ground to the junction between resistor R2 and capacitor C1. Capacitors Cl and C2 are adjustable for tuning purposes.

Phase correction is provided by the use of tube 5! and its cathode impedances as an inductive load inshunt with the circuit of the driver tube '2; The principle is simply that if an inductive current is drawn off the driver tube 2, which is equal;t0, the capacitivecurrent drawn bythe input'circuit network fortube I, then tube-2 operates at unity power factor. The necessary parallel inductance could, of course, be constituted as a simple winding on amagnetic coil, but at low frequencies several hundred henries would be required. With the connections as shown, the

inductance L can be very much smaller. One henry' was used in testing an embodiment of my invention and it was found that this method had the added advantage that the inductive current could be varied over a wide range by changing the value of the resistor R5. The equivalent inductive load added by tube 5| has a high equivalent series resistance and very low Q. This makes the tuning of the apparent inductance across the network very broad, so that the setting of the resistance value of R5 is not critical. When resistor R5 is adjusted to the center of the tuning range, the only. noticeable effect of a change in the-resistive value or in constants ofthe tube 5| isto produce a slight change in the amplitude of oscillation.

It can be shown mathematically that the current is traversing the auxiliarytube 5i and the current'is traversing the input circuitnetwork for tube I may be so balanced that the circuit of tube Zoompletely drops out of the frequency equation and the network then has complete control of frequency. 'It should be emphasized, however, that the inductance L should have a very low Q so that no substantial tuning results. Otherwise this coil itself would enter into the frequency equation and undo the work it Was put there to accomplish. At high frequencies, such a coil presents no problem, but at low audio frequencies, even though the Q is low, it would become very large and difficult'to adjust unless the tube 5| were provided for amplification. The circuit of tube 5| will, therefore, act as a high inductance in series with a large resistor, which is the effect desired, and at the same time the adjustment is readily'made over a large range by varying the value or resistor R5. The required inductance of the-coil L is-also reduced almost in direct proportion-t0 the gain of tube 5!.

It will be appreciated by those skilled in the art that my invention-is capable of modification in various ways to meet different requirements for which an R. C. oscillator would be used. I do not intend, therefore, to be limited to the exact details of construction of the several embodiments herein shown and described.

I claim:

1. In an oscillation generator, a plurality of electron discharge devices each having a cathode, an anode and a control electrode, a direct current source having circuit connections from its negative terminal to ground and from its positive terminal to said anode electrodes, a potentiometer in the anode circuit of one said device, a substantially non-inductive feed-back circuit connected between a point on said potentiometer and ground, said circuit having a predetermined time constant, the control electrode of a second said 'device being connected to a point on said feedback circuit-which divides the same intotwo substantially'equivalent impedances, a common refor sustaining the oscillations generated and for maintaining substantially a zero degree phase shift in saidfeedback circuit.

' 2. oscillation generator comprising two triode discharge tubes having a'common impedance connectingtheflcathodes to ground,- onenof said tubes having its control grid directly grounded and having a substantially noneinductivev circuit impedance. connected between its anode and the positive terminal of a direct;current source, a ground connection for the negative terminal of said source, a non-inductive time constant network connected between said anode circuit impedance and ground, and means for controlling the grid of the other, said tube, by feedback energy traversing said network,=the grid control potential being derivedfrom apoint on said network at whichno phase shift exists with respect to the anode potential of the first. said tube.

3. An oscillation generator having two triode discharge tube stages, a non-inductive time constant network connected from theoutputside of the second stage to the input side of the first stage and thence to'ground for producing regeneration, a control grid in the tube of each stage, the control grid of the firststage being connected to ground through aportion of said time ,constantnetwork, the control grid ,of the second stage having a resistivegroundconnection which isv shunted by a capacitor,;said network having a plurality of alternatively usable resistance, elements eachoperative in a different tuning range and gang-controlled tuning condensers for fixing the oscillation frequency within any selected tuning range, and means including a common impedance connecting the cathodes of both tubes to ground for causing the tube of the first stage to exercise control over that of the second stage.

4. An oscillation generatorpcomprising at least two electron discharge device's each' having electrodes including a cathode, an anode and a control grid, a direct current source having, circuit connections from its positive terminal ,to said anode electrodes, a resistor-in the anode circuit of one of said devices, a feedback circuit connectedibetween the anode of said one device and ground, said feedback circuit having a predetermined time constant and including a connection at a suitable point thereof to the control grid of the other of said device for providing substantially a zero degree phase shift in the control of said grid of said other device by output voltage from the said one device, a common impedance connecting the cathodes ;of the two said devices toground, a conductive connection from the control electrode in the said one device to ground, and means including circuit components lfor said generator whereby a gain factorofvat least. 3 is maintained.

5. An oscillation generator according to claim 4 and having a gain control device including a diode rectifier tube the anode of which is connected to the control electrode of said one device and the cathode of which is subject to control by output energy from a utilization device, said utilization device being coupled to the output circuit of said generator.

6. An oscillator comprising a first and a second triode discharge device, a direct current source, circuit connections from said source to the electrodes of each said device and to ground, said connections including anode circuits through which positive potentials are applied to the anodes in said devices, the anode circuit forthe second device containing a potentiometer, a non-inductive feedback circuit including time constant components comprising series resistance and capacitance connected between a point on said potentiometer and th control grid of the first device, a connection having parallel resistanoe and capacitance from the grid of the first device to ground, a single resistor connecting the cathodes of the twodevices to ground, a conductive connection from the control grid in the second device to ground, and volume control means operative upon the last mentioned control grid to producefrequency stability by preventing overdrive of the oscillating system.

7. An oscillation generator as recited in claim 4 including a threshold bias circuit for the grid of said one device comprising adiode rectifier tube connecting the control grid of said one device to an adjustable source of potential.

8. In apparatus for generating oscillations of substantially fixed frequency and substantially constant amplitude, two electron discharge. tubes each having input electrodes including a grid and output electrodes, an outputcircuit coupled with the output electrodes of one of said tubes, a resistor connected to the output electrodes of one of said tubes, a resistiveand capacitive feedback network coupling the output electrodes of said one of said tubes to the input electrodes of the other of said tubes to provide regenerative feedback in said tubes, the impedance of said first resistor being small relative to the impedance of said network, resistive means included in said network connecting the grid of said other of said tubes to ground, a common cathode resistor coupling the cathodes of bothoi said tubes to ground and a biasing circuit for the control grid of said one of said tubes such as to produce in saidtubes a gain of at least three.

9. In apparatus for generating oscillations of substantially fixed frequency and substantially constant amplitude, two electron discharge tubes .each having electrodes including an anode, a

cathode and a control grid, an output circuit coupled with the anode of one of said tubes, series resistors and capacitors in a feedback circuit coupling the anode of one of said tubes to the control grid ofthe other of said tubes, said capacitors and resistors being proportioned to 'feed voltage of the generated frequency from the anode of said one tube. to the control grid of said other tube without materially shifting the phase of the said voltage, a resistor connecting the grid of the otherof said tubes to ground, a common cathode resistor coupling the cathode of both of said tubes to ground to maintain the same instantaneous voltage on the cathodes of both tubes and a biasing circuit for the control grid of theother of said tubes such as to produce in said two tubes a gain suificient to maintain continuous oscillation in the two tubes.

10. An oscillator as recited in claim 6 in which said volume control means includes a voltage divider in shunt to a direct current source, and a rectifier connecting a point on said voltage divider to the control grid of said second discharge device.

11. In apparatus for generating oscillations of substantially fixed frequency and substantially constant amplitude, two electron discharge tubes each having electrodes including an anode, a cathode and a control grid, an output circuit coupled with the anode of one of said tubes, a resistor and capacitor in series in a feedback circuit coupling the anode of one of said tubes to the control grid of the other of said tubes,said capacitor and resistor being proportioned to feed voltage of the generated frequency from the anode of said onetube' to the oontrol grid of said other tube without materially shi'fting the phase of the said voltage, a resistor in said feedback path connecting the grid of the other of said tubes to ground, a common cathode resistor coupling the cathode of both of said tubes to ground to maintain the same instantaneous voltage on the cathodes ofboth-tube's, a biasing circuit for the control grid of said oneof said tubes such as to produce in=said--two tubes again sufficient to maintain continuous oscillation in the two tubes and means for compensating the effects of reactive currents drawn by the input of said other tube-comprising a third tube having an output in series with a r'eactance in shunt to the output ofsaid one tube, said last named rea-ctance being ofa value and sign such that said third tube draws-reactive current or an opposite sign with respect tothe reactive current drawn by the inputofthe other tube and of amagnitude which compensates the said reactive current drawn by the input of the other tube.

12; Inapparatus for generating oscillations of substan'tially'fixed frequency and substantially constant amplitude, two electron discharge tubes each having input electrodes including 'a grid and outputelectrodes; an output circuit coupled with the output electrodesof one of said tubes, a resistor connected tothe out-put electrodes-of one of said tubes, aresistiveandcapacitive feedback network coupling the output electrodes of said one of said tubes tothe input electrodes of the other of said tubesto provide regenerative feedback in said tubes, resistive means included in said network connecting the grid of said other of said tubes to'ground', a common cathode resistor coupling the cathodes of 'both of said tubes to ground and a biasing circuit for the control grid of the other of said tubes comprising a resistor between its grid and cathode, a rectifier circuit including said resistor as a rectifier load and connections for applying the oscillations generated tosaid rectifier;

13. In apparatus for generating oscillations of substantially fixed frequency and substantially constant amplitude, two electron discharge tubes each having input electrodes including a grid and cathodea-nd outputelectrodes including ananode and said cathode, an output circuit coupled with the output electrodes of one of said tubes, a variable resistor connecting a positive potential sourceto the output electrodes of one of said tubes, aplurality of'resistive and capacitive elemen-ts in feedback networks, switching means for coupling the output electrodes of said one of said tubes tothe input electrodes of the other of'said tub'es through one of said networks: to provide regenerative feedback in said tubes, the resistive means included in said networks also connecting the grid of said other of said tubes to ground, a common cathode resistor coupling the'cathodes' of both of said tubes to ground and a biasing circuit for the control grid of said one of said tubes such as to produce-in said tubes again of at least three.

14. In apparatus for genera-ting oscillations of substantially fixed frequency and substantially constant amplitude, two electron discharge tubes eachhaving input electrodes including a grid and output electrodes,- an output circuit coupled with the output electrodes of one of said tubes, a potentiometer resistor connecting the output electrodes of one of said tubes to a source of direct current potentiaLa resistive and capacitive phase shifting network coupling a point onsaid potentiometer resistor to the input electrodes of the other of said tubes to provide regenerative feedback in said tubes, resistive means included in said network connecting the grid of said other of said tubes to ground, a common cathode resistor coupling the cathodes of both of said tubes to ground and a biasing circuit for the control grid of said one of said tubes comprising a rectifier in a rectifier circuit including av resistor between the grid and cathode of said one tube and means for impressing generated oscillation on said rectifier circuit.

M UR CE T REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Proceedings IRE, vol. 27, No. 10, Oct, 1939, Pp. 649-655, article by Terman et al.; ibid, vol. 29, No. 2, Feb.'1941, pp. 43-49., article by Ginzton et al-.';- ibid., vol. 31, No. 1-, Jan. 1943, pp, 22-25,

article by Chang. 

